Here is the code:

/* Main Menu */

.MainMenu_MenuContainer{ background:transparent; width:100%; }
.MainMenu_MenuBar{}
.MainMenu_MenuIcon{ display:none; }
.MainMenu_RootMenuArrow{ display:none; }
/*.MainMenu_MenuArrow{ display:none; }*/
.MainMenu_MenuItem,.submenuitembreadcrumb{
 color:#484848; font-size:11px; font-weight:normal;
font-family: Tahoma, Arial, Helvetica;
 padding: 2px 10px 3px 5px;
 /*border:1px #B2B2B2 solid; border-width:1px 0px;*/
}
/*.MainMenu_MenuArrow{
 padding:0 3 0 7;
 border:#484848 solid; border-width:1 1 1 0;
 text-align:right;
}*/
.submenuitemselected{
 color:#db5113;
 font-size:11px;
 font-weight:normal;
 padding:2px 15px 2px 10px;
 border:1px #fff solid;
 border-width:1px 0px;
 Margin:2px;
 background:#FFFFFF;
}
.submenu{
 z-index:1000;
 border:1px #CDCDCD solid;
 padding:0px;
 margin-top:2px;
 margin-left:2px;
 background:#fff;
}
.rootmenuitem TD{
 color:#db5113;
 height:25px;
 font-size:12px; font-weight:bold;
 padding: 2px 10px 3px 5px;
 background:transparent;
/*border-bottom: #D9D9D9 1px solid;
border-top: white 0px solid;
border-left: #D9D9D9 1px solid;
border-right: #D9D9D9 1px solid;*/
}
.rootmenuitemselected TD,.rootmenuitembreadcrumb TD{
 color:#484848;
 height:25px;
 font-size:12px; font-weight:bold;
 padding: 2px 10px 3px 5px;
 background: #FFFFFF;
 background-color: #FFFFFF;
/*border-bottom: #D9D9D9 1px solid;
border-top: white 0px solid;
border-left: #D9D9D9 1px solid;
border-right: #D9D9D9 1px solid;*/
}
.MainMenu_MenuBreak{
width:2px; 
border:0px;
 background:transparent;
}
.MainMenu_MenuBreak TD{
        width:2px;
 color:#ffffff;
}

.MainMenu_Active{
 background: #ECE9D8;
 color: #db5113;
 height:25px;
}
.MainMenu_BreadcrumbActive2
{
 HEIGHT: 25px;
}

/* END Main Menu */

OK, so i have the same problem and here is what I found: 

   1. I commented out the menu styles and the problem persisted
   2. I commented out the styles that were overriding default.css, the problem persisted, so
   3. I commented out my page styles and the problem went away.

Great so somewhere in here was the problem line(s).  after much commenting, uncommenting i finally found the problem.  I have a div that wraps around all of the content which I use to center the content and set background styles for the content area of the site, somewhat common I imagine.  I have a style like this:

.ContentWrapper { width: 800px; background: white;}

If I remove the width setting everything is fine.  As soon as I constrain the width of the content wrapper the problem returns.    My first thought is something is bigger than the 800px so let me remove it.  Can't find a darn thing that should be spilling outside of that setting, however when i define other widths i just set them to 100%.  I want the tables, divs etc to fill the content areas so I set them this way. 

All that to say, I know that forcing the width causes the problem to occur, but I don't know why!  Any suggestions would be great.  Hopefully this will help find the cause of this problem. 

Just to rehash the problem:

IE7 with a DNN Site 4.5.x or above when not signed in, the page jumps around when you mouse over a main menu item that has a submenu.